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Albert Einstein once said, "Intellectual growth should commence at birth and cease only at death." And these days, experts on aging agree. Studies continue to show that lifelong learning is the key to keeping our minds sharp and our brains strong, an important contributor to wellness overall, as we grow older. "Cognitive decline is not a normal function of aging," insists Andrew J. Carle, executive in residence and the founding director of the program in senior housing administration at George Mason University in Fairfax, Va. "The brain doesn't have to be in decline. You can keep it strong; you can exercise your brain."

For many years, the idea of cognitive calisthenics conjured up images of silvered-hair retirees playing bridge or noodling over the New York Times crossword puzzle, ideally while relaxing by the pool. While those activities certainly do contribute to brain health, they're only the tip of the iceberg of what's available to mature adults who want to educate themselves in effective, enjoyable, and, in some cases, luxurious, ways. From downloading Harvard lectures to their computers, to spending a semester at sea with a ship full of students and professors, options for lifelong learners abound. Here are five of the best:

Lectures in the Living Room

You don't have to leave home to exercise your brain -- just pop a disc into your CD or DVD player, or download a digital video. The Great Courses of Chantilly, Va., formerly known as the Teaching Company (thegreatcourses.com), makes and sells video and audio recordings of some 390 classes, taught by professors of top universities, on everything from literature to math to wine-tasting. Best sellers include "Understanding the Universe: An Introduction to Astronomy" ($229.95 on DVD); and "How to Listen to and Understand Great Music" ($699.95 on DVD).

For an unbeatable Brit fix, head to Oxford for a one-week session for adults.
Glyn Thomas/age fotostock

Another option: The new online platforms that allow anyone with an Internet connection to enroll in select college classes, for free. The most comprehensive is Coursera (coursera.org), which has partnered with 34 major universities, including many Ivies, to bring users access to 200 courses, ranging from "Jazz Improvisation" with Grammy winner Gary Burton through the Berklee College of Music, to "Modern & Contemporary American Poetry," via the University of Pennsylvania. Also: Udacity (udacity.com), which offers 18 courses in computer science, math, and business; and edX (edx.org), which has a handful of classes from Harvard, MIT, UC Berkeley, and the University of Texas System.

Heading Back to College

For many, there's no substitute for classroom learning. Luckily, that's fairly easy to obtain, no matter where you live or what your budget is.

According to the American Association of Community Colleges, 84% of the nation's community colleges offer programs specifically designed for those 50 and older -- and most waive tuition. One standout: Brooklyn-based Kingsborough Community College's "My Turn" (www.kbcc.cuny.edu/myturn/Pages/KCCMT.aspx), which lets students over 60 take classes on a space-available basis in subjects ranging from fashion to literature. The New York college's free program also includes activities such as tours of New York's Grand Central Station and Jacob Riis Park.

Many four-year colleges, particularly those that are state funded, also welcome local retirees to their classrooms, tuition-free. Penn State University's Go-60 program (ceup.psu.edu/lifelong-learning) allows seniors to do anything from audit a course to earn a degree, at no cost, while Boston University's Evergreen center (bu.edu/evergreen/) lets adults 58 and older audit classes for $160. If you'd rather go back to school with people your own age, nearly 120 colleges nationwide now boast Osher Lifelong Learning Institutes, where students 50 and older take classes designed specifically for "seasoned adults" (usm.maine.edu/olli, for a listing of one near you). Among the largest: the 35-year-old OLLI at Duke in Durham, N.C. (learnmore.duke.edu), where members pay $35 in annual fees to access some 300 courses on topics ranging from Shakespeare's Richard II to "Psychic Experiences," for $40 to $85 apiece.

Frisbee, Anyone?

If you're looking for what George Mason's Carle calls "a more intergenerational experience," try a University Based Retirement Community, which offers living quarters practically on campus and real immersion in student life. "In the good ones, you get the equivalent of a student ID," Carle says, enabling residents to borrow from the library, audit classes, and use the campus recreation center. Meanwhile, university students work in the retirement community through internships, part-time jobs, and theater performances.

There are currently about 50 UBRCs, with dozens more on the way. Capstone Village at the University of Alabama in Tuscaloosa (capstonevillage.com) is among the newest. Campus living is not cheap: one-time entrance fees can range from $160,000 to $600,000, with monthly costs of $2,000 to $7,000, depending on an individual's level of care, amenities and services such as housekeeping. But Carle, for one, is a big fan. "If you put people in an environment from their 20s, they will behave younger" -- and feel healthier, too.

Hitting the Road

Educational travel programs have come a long way since 1975, when Elderhostel first began offering noncredit college courses to retirees who lodged in school dormitories in distant locales. These days, learning trips are likely to be multigenerational -- you can bring the kids and grandkids -- and feature action-packed itineraries, optional private excursions and, in some cases, plush accommodations. Elderhostel, recently rechristened Road Scholar, (roadscholar.org) now arranges more than 6,000 tours in 150 countries, including hiking adventures and bike journeys. The lodging is usually quite comfortable.

University alumni associations, which once offered travel only to their school's graduates, are increasingly welcoming all comers to their expeditions, which typically feature faculty members who lecture on the trips. While there can be tremendous similarities among different schools' offerings -- many rely on the same small group of tour operators to create their packages -- one standout is Stanford's travel/study program (alumni.stanford.edu). Nearly 100 faculty leaders participate in the university's offerings, which for 2013 include ringing in the new year at the Tsar's Ball at Catherine's Palace in St. Petersburg. The trip, which features curator-led tours of the Hermitage and the Yusupov Palace, where Rasputin met his end, costs $8,495 per person.

Museums are another rich source of educational vacations. The American Museum of Natural History (amnhexpeditions.org) has been at it the longest, arranging and leading study and adventure tours since 1953. Among its most popular is its trip to Patagonia, which Elizabeth DeGaetano, associate director of AMNH Expeditions, says it plans to offer again in early 2014. The voyage will feature a stay at a new luxury hotel, Tierra Patagonia Hotel & Spa, in Torres del Paine, Chile, and a three-night cruise, with stops at Cape Horn National Park and Magdalena Island to see the penguins. For those itching to travel in 2013, the museum's new offerings include a 12-day journey to Burma with two anthropologists, and a 10-day cruise of the Mississippi River and Gulf Coast, accompanied by a geologist and a trio of musicians from the Smithsonian Jazz Masterworks Orchestra. Prices start at $6,390 for Burma and $4,995 for the Mississippi cruise

Philip Barnes, a British pioneer of educational tourism, died in 2009, but the organization he founded, ACE Cultural Tours (aceculturaltours.co.uk), lives on. Known for small, expert-led tours and cruises with themes like "Land of the Etruscans" and "The Rome of Caravaggio & Bernini," ACE wins kudos from Anglophiles for its work with English Heritage, the U.K. government's advisor on the historic environment, to offer excursions focused on such topics as "Medieval Suffolk and East Anglia: Romans and Saxons." Visitors to England can also take advantage of ACE's one-day courses, which include lectures and tours such as "Handel in the City" and "Tudor & Stuart Country House Craftsmen," with many priced at less than 100 pounds, or approximately $159.

Anglophiles might also want to check out the Oxford Experience (oxfordexperience.info), a summer school of one-week courses held at the famed university's Christ Church college, which dates to 1524. Participants live in the school's dormitories, dine in its Great Hall (a.k.a. Hogwart's Hall in the Harry Potter films), and meet in small groups for classes ranging from "The Black Death" to "Mythical Animals: Their Evolution Past and Present." Many courses boast optional excursions in and around Oxford. Weekly fees start at £1,180.

Ahoy!

If you're the nautical type, check out Semester at Sea (semesteratsea.org), a traveling campus, complete with a 9,000-volume library, run by the University of Virginia. Adults 40 and older can set sail with college students on multimonth voyages that explore the Atlantic, travel through Europe, or even go around the world -- all while attending undergraduate courses right on board. Rates start at $16,242 for summer semester voyages. Not up for such a long journey? The same ship offers shorter "enrichment voyages," which feature interactive lectures and workshops, such as its 25-day Origins and Empires cruise to the Caribbean and Central and South America in December. But be warned, says alternative-travel specialist Alison Gardner: Each voyage is so stimulating "you really don't want it to end."